Assigning my rigged model to a Mixamo skelton
Create a copy of your rigged character, go to the first frame (or a rest pose frame, if it does exist), use your rig to copy the pose of the Mixamo rig as close as possible, then go to the modifiers and apply the armature modifier.
Not necessary, but if you want to stay clean at this point you can delete the rig and all vertex groups.
Select the character, shift select the Mixamo rig and press Ctrl P (Automatic weights).
Then you can turn on and off the visibility of the two versions of your character.